Prior private branch exchange (PBX) systems provided consecutive numbering of subscriber stations and, for simplicity, also maintained a correspondence between station numbers and switching equipment locations. This structure was simple but inefficient since gaps in the numbering plan left large blocks of corresponding equipment unused. The use of special dial codes, such as "0" for an attendant or "9" for an outside line, precluded the use of those digits as a first digit in station codes, thus inefficiently using the equipment's capability. For example, a station number such as 9302 could not be used within the PBX system. Upon receiving the station number 9302, the PBX system would generate a dial tone because the first digit "9" is interpreted as a special dial code for an outside line. Consequently, the exclusion of station numbers beginning with digits "0" or "9" eliminates the use of two thousand potential station numbers.
Subsequent telephone switching systems used flexible numbering arrangements which arbitrarily associated a station number with an equipment location. This arrangement efficiently makes use of the equipment since each equipment location is assigned an arbitrary station number and no blocks of equipment need be unused because of gaps in the numbering plan.
One such prior art flexible numbering arrangement is disclosed in U.S. Pat. No. 2,585,904 which issued to A. J. Busch. The Busch patent discloses an apparatus which utilizes the first of four dialed digits to select one of ten available groups of station equipment. Each of the subsequent three dialed digits are translated and used to activate a respective code point within the selected group. The code points are cross-connected to define an equipment location associated with the particular combination of three-dialed digits in each of the ten available groups of station equipment. Thus, the first-dialed digit is used to select the particular group while the last three-dialed digits enable a particular equipment location in each of the ten available groups, yielding a specific equipment location for each dialed four-digit number. However, this efficiency was obtained at the expense of providing an extensive cross-connection arrangement since several cross-connections are needed to define the correspondence between each station number used and the associated equipment location.
Consequently, there is a need for a numbering system which utilizes a full compliment of dialing digits to establish a switching connection without using an extensive cross-connect arrangement.